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Citrus vegetative growth manipulation is an effective tool to manipulate carbohydrate partitioning within the tree during phases where it may be critically needed in other processes, such as fruit set or fruit growth, rather than for shoot elongation. Recent literature reporting on citrus shade netting, found an increase in vegetative growth in reaction to reduced light levels, which indicates that vegetative growth control may become an increasingly important practice as the use of shade netting increases (Wachsmann et al., 2014). Various practices can be used to manipulate vegetative growth, however, for the purpose of this study, only vegetative growth control by means of PGR’s will be reviewed.

The mode of action of these PGR’s are related to endogenous GA synthesis of the tree. Most of the known growth retardants reduce vegetative growth by disrupting the pathways of GA synthesis, thus partially retarding the stimulating effect of GA on cell elongation and vegetative growth (Smeirat and Qrunfleh, 1988). Various growth retardants have been investigated in citrus and other fruiting crops, however most of these substances proved to have inconsistent and unreproducible results (El-Otmani et al., 2000). Later research on citrus and avocados indentified the triazoles paclobutrazol, uniconazole, and prohexadione-calcium as the gibberillin-biosynthesis inhibitors producing the best results (Greenberg et al., 1992; Le Roux and Barry, 2010; Penter and Stassen, 1998).

Greenberg et al. (1992) found that PB sprays and soil application during autumn increased the number of flowering shoots sprouting in the spring, accompanied by a reduced number of vegetative shoots. The influence of PB on shoot elongation evaluated in this study, showed that the spring and early summer PB treatments on ‘Minneola’ tangelo gave the best results in reducing excess elongation of summer shoots. The tree height was also evaluated with ‘Minneola’ tangelo trees topped to similar height before the PB treatments. After six

months it was found that the 1000 ppm PB treated trees were roughly 400 mm shorter than the control trees, confirming the effect of PB on retarding vegetative growth of citrus trees (Greenberg et al., 1992).

Uniconazole and prohexadione-calcium (ProCa) research on vegetative growth of potted ‘Eureka’ lemon nursery trees showed that 1000ppm uniconazole returned the best results for retarding shoot growth, followed by ProCa at 800ppm (Le Roux and Barry, 2010). Interestingly, the number of nodes on the longest shoot did not differ from the control, while the node length differed significantly, with ProCa and uniconazole having the shortest nodes (Le Roux and Barry, 2010). This indicates that these two growth retardants did not reduce shoot length by altering the number of nodes, but rather by reducing the internodal length. This suggests that the number of nodes, from which inflorescence can sprout in the following season, was not reduced.

Increased fruit size and flowering were reported for avocado and citrus, respectively in reaction to the application of growth retardants (Greenberg et al., 1992; Penter and Stassen, 1998), thus resulting in higher crop value. However, Greenberg et al. (1992) found that on citrus, early spring and summer PB sprays had a negative effect on fruit development by shifting the fruit size distribution to a smaller average fruit size. Contradicting results were found for uniconazole on avocados, where inhibition of the shoot growth flushes during the fruiting season lead to an increase in average fruit size (Penter and Stassen, 1998).

According to literature, uniconazole, ProCa and paclobutrazol produced the best results in retarding vegetative growth, however it is unlikely that paclobutrazol will be registered commercially on citrus due to the negative imapact on fruit size and its persistance in the enviroment and the plant (Le Roux and Barry, 2010).

2.5.2 Fruit set

Fruit set in citrus is a tightly regulated physiological process, which is regulated by numerous factors such as carbohydrate status, endogenous GA’s, bearing unit quality and other cultural practices (Talon et al., 1990). In commercial citriculture however, the best results in enhancing fruit set is obtained with exogenous GA3 application during full bloom (Krezdorn,

1969).

The mechanism behind the promoting effect of GA3 on citrus fruit set is an intensively

researched field and some of the first results indicated that it is responsible for enhancing early fruit growth which leads to an inhibition of fruit abscission and thus an increase in fruit set (El- Otmani et al., 1992). García-Martínez and Garcia-Papi (1979) however reported that foliar application of GA3 resulted in increased mineral nutrient translocation to the developing

fruitlets, while Mauk et al. (1986) reported that the application of foliar GA3 sprays increased

the sink strength of developing ovaries resulting in increased carbohydrate translocation and a transient increase in fruit set.

Application of GA3 during full bloom is inevitable in the production of parthenocarpic

and self-incompatible cultivars (low endogenous ovary GA levels), and in areas where decreased fruit set is experienced (García-Martínez and Garcia-Papi, 1979). GA3 applications

is especially important in areas producing ‘Clementine’, as this cultivar is prone to high ovary abscission during the post bloom period (El-Otmani et al., 2000). El-Otmani et al. (1992) also did extensive research on the timing and concentration of GA3 as a foliar application to increase

‘Clementine’ fruit set and final yield. To ensure maximum coverage of as many ovaries as possible it was concluded that during sparse flowering seasons GA3 should be applied twice,

at lower concentrations during early bloom to petal drop. However, in seasons with a shorter bloom period, single sprays with increased dosage, showed promising results.

When GA3 is applied to increase fruit set, the best results are obtained when a wetting

agent is added, and the spray covers the tree until the point of runoff (El-Otmani et al., 2000). However, on hot days with high evaporative potential and mixtures with high concentrations, should be avoided as cases of new shoot dieback and leaf drop have been reported (El-Otmani et al., 2000).